The recombinant production of human proteins is generally performed by cultivation of stably transfected eukaryotic and preferably mammalian cell lines and isolation of the protein from the culture broth. In case the recombinant proteins are intended for pharmaceutical applications, it was for a long time general practice to employ non-human cell lines in order to exclude the risk of copurifying infectious agents which may be harbored and expressed by human cells.
In the production of some human proteins, such as human blood clotting factor VIII, the use of non-human cell lines were found to entail certain disadvantages, e.g. unsatisfactory secretion levels of the expressed protein into the medium. It is believed that this may be due to slight differences within different types of mammalian cells concerning intracellular pathways for protein translation and modification, which also might have an effect on the biological activity of the expressed polypeptide. Apart from this, there were concerns that therapeutic proteins purified from non-human expression systems are contaminated with cellular components which can give rise to antigenic reactions in the patients. Also a concern was the non-human glycosylation pattern found on human proteins recombinantly produced in non-human expression systems. It is thought that this increases the likelihood of antigenic reactions in the patient. Furthermore, biological stability and efficacy of blood proteins such as clotting factors is substantially influenced by their N-glycosylation pattern. Especially peripheral and terminal monosaccharides are important, because they are detected by specific receptors from cells which are responsible for their degradation. Clotting factors, for example, carry sialic acid residues as terminal monosaccharides. Modification on the composition of sialic acids in the antennae of glycoproteins can result in heterogenous glycosylation patterns. Thus, biological stability and efficacy is crucially involved when modification occurs. Hence, it is an important consideration in the production of recombinant clotting factors to evaluate the influence of glycosylation from non-human production cell lines versus human cell lines.
On the other hand, general methods for high level protein expression of a desired gene comprising immortalized, stably transfected mammalian cell lines expressing viral transcription activator proteins were made available (e.g. U.S. Pat. No. 5,712,119). These cell lines can be transformed with a vector construct where a suitable viral transcription promoter is operatively associated with a DNA sequence defining a gene of interest, the transcription activator proteins provided by the cell lines activate the viral transcription promoter and hence initiate the expression of the gene of interest. As important as the cell line is the vector used for the introduction of the recombinant gene into an immobilized production cell line. A wide variety of vectors were utilized for translation of mammalian proteins, (for example, Witsch-Baumgartner, M et al. Am. J. Genet (2000). 66, 402-412 cloned DHCR7 cDNA into pCI-neo mammalian expression vector and expressed in the HEK 293 cells; McGarvey, T. W. et. al. Oncogene (2001) 20, 1041-1051 cloned TERE1 gene into the pTARGET mammalian expression vector and expressed in the human bladder transitional cell carcinomas; and Lin Lin et. al. J Biol Chem (2002) 277 (44) 41872-8 cloned the AchR gene into mammalian cell expression vector pEF6/myc-His vector and expressed it in 293 cells). A recently developed very potent vector which has proven to be capable of over-expression of recombinant proteins is the so-called pcDNA™3.1 vector of Invitrogen. Li J. et al., Life Sci. 2004 Apr. 16; 74(22):2693-705 have successfully over-expressed histone deacetylases using pcDNA 3.1 in HEK 293 cells. The cells were stably transfected and cultured in the presence of serum. Yuan-Gen Fu. et al., World J Gastroenterol 2003 have produced recombinant Caspase-3 using a pcDNA 3.1(+) based eukaryotic vector on gastric cancer cell line SGC7901 transiently transfected with said vector and cultured in the presence of serum. Ma H. et al., Invest Ophthalmol Vis Sci. 2000 December; 41(13):4232-9 examined the lack of stable protein and loss of enzymatic activity expressing Lp82 and Lp82-related proteins subcloned into pcDNA3.1 vector using COS-7 as cell line. The cells were transiently transfected and cultured in the presence of serum in the medium. Thioredoxin overexpression prevents NO-induced reduction of NO synthase activity in lung endothelial cells. Zhang J. et al., Am J Physiol. 1998 August; 275(2 Pt 1): L288-93 disclose the overexpression of thioredoxin gene in cultured porcine pulmonary artery endothelial cells by transient transfection of these cells with pcDNA 3.1 vector. The transfected cells were cultured in medium supplemented with serum. Shinki T. et al., Proc Natl. Acad. Sci. USA 1997 Nov. 25; 94(24):12920-5 compared a full length cDNA for the rat kidney mitochondrial cytrochrome P450 mixed function oxidase, 25-hydroxyvitamin D3-1alpha-hydroxylase with vitamin D-deficient rat kidney cDNA and subcloned it into mammalian expression vector pcDNA 3.1 (+) and transiently transfected the vector into COS-7 transformed monkey kidney cells. The transfected cells were cultured in medium supplemented with serum. Zhang et al., Acta Biochimica et Biophysica Sinica 2004, 36(10): 707-712 disclose the transfection of human embryonic kidney 293 cells with pcDNA containing a gene coding for the humanized 520C9 single chain Fv antibody/human interleukin-2 fusion protein. Supernatant was taken after having cultured the cells for three days in serum-free SFM II media. The resultant fusion protein possessed binding specificity against p185 (promising target for antibody therapy in breast cancer) and retained the important immuno-stimulatory activities of IL-2. Chen, J. Z. et al., Int J Biochem Cell Biol. 2004 August; 36(8):1554-61 over-expressed Bim proteins, which are essential factors for apoptosis, using HEK 293 cells transfected with pcDNA-Bim alpha3.
A further measure for increasing the safety of recombinant proteins for pharmaceutical applications is the use of serum-free medium in the culturing process, as the use of serum represents a safety hazard as well as a source of unwanted contaminations. Such serum-free cultivation has the drawback that the yields of the production process are generally significantly reduced. A further safety concern is the use of serum when transfecting the host cells as a regular way in the practice, as the use of serum in the transfection procedure may cause unwanted biological material to be integrated into the cells which later on contaminated the product expressed by the cells in the production process. While some of the available methods for the production of recombinant proteins (including those mentioned above) do allow serum-free cultivation, serum-free stable transfection of human cells is not known. In the 19th ESACT Meeting, Harrogate, 5-8 Jun. 2005 the serum free transfection of CHO cells was suggested by Kuchenbecker et al.
Thus, it is desirable to develop an effective and safe method to produce human recombinant proteins.